Spring and winter, just 35 miles apart
What a zany morning around the Puget Sound area, where one area was basking in mid-upper 40s while just 35 miles away, it was well below freezing amid a dense fog.
Welcome to winter in the Northwest.
There was quite a dichotomy around the Sound due to a warm east wind that kept temperatures relatively mild in some spots, while wind sheltered spots denied the breezes' warmth instead got the full wrath of Jack Frost.
At 6 a.m. Friday, the temperature in North Bend was 47 degrees, while 35 miles away in Tacoma it was 27 and foggy (and even down to 25 in Olympia). But another head scratcher -- that wind in North Bend is coming out of the east from Eastern Washington-- where the temperature was also in the upper 20s Friday morning.
How did this all happen? It's quite a complex situation.
In the summer time, this pattern would mean a nice warm day everywhere, likely several degrees above normal and perhaps in shouting distance of a record high. But in winter, the shorter days and the arctic tundra that is Eastern Washington both play a factor to keep some spots much colder than the others while others get even warmer.
Let's start with Tacoma and Olympia and why they're in the 20s with fog. With the long, clear nights, the day's heat is easily radiated back out into space, allowing temperatures to plummet until they reach the dew point -- in the mid 20s right now -- creating dense fog. Cold air is denser than warm air, so as the temperature drops, it creates a cold dome of air that is relatively "heavy" and just sits there, keeping them enveloped in the cold and fog.
A nice wind would help to mix in some warmer air and devour the fog, but the topography of the South Sound is such that when the wind blows from the east, it's blocked and their air remains stagnant. It means areas like Tacoma and Olympia can be stuck in the fog for much of the day with temperatures struggling to get into the 30s and maybe low 40s.
Eastern Washington suffers the same fate -- clear nights allows temperatures to drop and a cold air dome to form and settle. Their topography is also somewhat of a bowl in Central Washington, helping to trap that cold air even better.
The denser cold air actually creates its own high pressure, and as the other parts of Western Washington that have some ventilation warm, we've got much higher pressure in Eastern Washington than Western Washington, and that large difference in pressure will create a gusty east wind through the mountains in the hope of equalizing that pressure difference.
You can see that in this weather chart - notice the cold blues in Eastern Washington and the warmer greens/yellows in Western Washington, with the black lines denoting surface pressure -- notice how close together they are along the mountains -- a sign of strong winds:
Now here's the most important cog in the machine -- as that east wind comes over the Cascade crests and sinks down the western slopes, it causes the air to warm up and dry out, giving a nice temperature boost to those in the east wind's path. So North Bend, Gold Bar and Enumclaw bask in mild temperatures while those sheltered from the wind get stuck in the cold.
Seattle proper gets enough east wind to also get some warmth and avoid the fog. In fact, at 11 a.m., it was 55 degrees in Seattle, which was warmer than 24 of the 30 days we had last April (!)
And finally, speaking of wind, the pressure difference is strong enough that a Wind Advisory has been issued until 3 p.m. for the Cascade foothills for gusts to 45-55 mph possible. Meanwhile at Seattle's Boeing Field, Tacoma, and Olympia were all reporting dead calm winds.
All under a simple ridge of high pressure. Most other places, this pattern would just mean a sunny and mild-to-warm day everywhere. But in the complex meteorological smorgasbord that is the Pacific Northwest, a "simple" ridge can be anything but.